Wedgits are appropriate for even much smaller siblings (no choking hazards!) and easy to put together into all kinds of fun and interesting towers and shapes. We also got a stack of cards with pictures showing structures to build in increasing levels of difficulty, rather like 3-D tangrams. S flew through the beginner levels, but I suspect that the more challenging ones will take some thought and perhaps parent assistance. Fun for everyone!
Also, judging from the "free play" that S has been doing with these all day they will be helpful in sequencing/size concepts as well as interesting balance/motor skills and spatial relations.